<!-- 字符串对象转换大写小写 -->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        // 字符串对象转换大写
            var str = "hello world";
        var strUpper = str.toUpperCase();
        console.log(strUpper); 

        // 字符串对象转换小写
        var str = "HELLO WORLD";
        var strLower = str.toLowerCase();
        console.log(strLower); 

    </script>

</body>
</html>